This invention relates to an electro-endosmosis type dehydrator for dehydrating the sludge formed, for instance, in a sewage-treatment plant, and more particularly to the structure of electrodes provided for a rotary drum in an electro-endosmosis type dehydrator in which the rotary drum is combined with a press belt.
Such an electro-endosmosis dehydrator in which a rotary drum is combined with a press belt has been disclosed, for instance, in Japanese Patent Unexamined Publication No. Sho-56-60604. FIG. 9 shows the configuration of such a rotary drum type and electro-endosmosis type dehydrator. In FIG. 9, the reference numeral 101 represents a rotary drum also functioning as an anode; 102, a water-transmissive press belt also functioning as a cathode and confronted through a sludge passageway with the rotary drum 101; 103 and 104, filtrating cloth belts laid over the dehydrating area surfaces of the rotary drum 101 and the press belt 102; 105, a DC source; and 106, a filtrated water receptacle.
In such a configuration, if a voltage is applied across the rotary drum 101 and the press belt 102, and sludge 107 is supplied to the sludge passageway while the rotary drum 101 is being rotated in the arrow direction, the sludge 107 suffers a mechanical press force, and at the same time the water contained in the sludge is charged into + as the result of an electric field acting therein so that the water flows to the cathode side, passes through the filtrating cloth 104 and the press belt 102, and drops into the filtrated water receptacle 106 so as to be drained out of the system. The dehydrated sludge is recovered as a dehydrated cake. A portion of the water which has been pressed and passed out to the rotary drum side by the mechanical press force is passed through the filtrating cloth 103, and drained out of the system through an drainage groove formed in the drum surface.
The above-mentioned conventional electro-endosmosis type dehydrator is, however, disadvantageous in the following points: That is, the rotary drum 101 and the press belt 102 also used as electrodes are covered with the filtrating cloths 103 and 104 which are electrically insulating substances. Accordingly, the effective area of the electrodes which are brought into contact with the sludge is decreased (a current flows from the electrodes to the sludge through liquid permeating the texture of the filtrating cloths), so that the conductivity between the sludge 107 and the electrodes is lowered. In this regard, sludge contacting with the electrode includes enough water so that high conductivity is maintained between the electrode and the sludge on the cathode side where water is collected by electro-endosmosis. On the side of the rotary drum 1 acting as an anode, on the contrary, water contained in the sludge is moved to the cathode side with the advance of the electro-endosmosis so that the water containing rate of the sludge contacting with the electrode surface on the anode side decreases so that the specific superficial conductivity is lowered. Hence, it is more and more difficult to flow a current, so that the electro-endosmosis flow quantity, that is, the dehydrating ability is lowered (the electro-endosmosis flow quantity is proportional to a current flowing in the sludge).
To prevent the conductivity between electrodes and sludge from lowering on the anode side, there has been such a trial that the electrode surface of the rotary drum is brought into direct contact with the sludge without using such a filtrating cloth, as disclosed, for instance, by Japanese Patent Unexamined Publication No. Sho. 60-25597.
However, such modified electro-endosmosis type dehydrator also has the following disadvantages: When the sludge is dehydrated with such electro-endosmosis type dehydrator, the sludge produces gas in the dehydrator by electrolysis. The gas thus produced is electrically non-conductive. If the gas is held detained between the sludge and the electrode surface during dehydration, it will obstruct the application of voltage to the sludge, thus lowering the sludge dehydrating performance on electrolysis. In this case, the gas produced on the side of the press belt (where water is collected) is discharged out of the system together with the water filtered; whereas, the gas produced on the side of the rotary drum (where no water is collected) is enclosed between the sludge and the electrode surface. Hence, it is essential to quickly remove the gas from the rotary drum side; more specifically, the gas produced between the sludge and the electrode surface must be quickly discharged out of the system to prevent the lowering of the performance of electroendosmosis dehydration.
For this purpose, another electro-endosmosis type dehydrator has been proposed under Japanese Patent Unexamined Publication No. Hei. 2-119906 in which a plurality of degasifying strings are laid over the cylindrical wall of the rotary drum.
The electro-endosmosis type dehydrator with the degasifying strings is, however, disadvantageous in the following points: The strings are belts which are laid over the electrode surface of the rotary drum and a pulley. Therefore, the strings are driven in such a manner as to cut into the sludge. Hence, when the sludge is conveyed along the sludge passageway while being compressed, the degasifying passageways formed by the strings may not work being clogged up with sludge, or the sludge often shifts the strings from their predetermined positions to the right or left. The strings are made of insulating material. Therefore, if the number of strings is increased to improve the degasifying performance, then the effective area of the electrodes which are brought into contact with the sludge is decreased as much; that is, the electro-endosmosis is lowered as much.